This invention relates to a method for treating a subterranean oil formation containing very viscous petroleum. It is well known to persons skilled in the art of oil recovery that many subterranean deposits of petroleum cannot be produced by conventional primary means because the viscosity of the petroleum is so high that virtually no petroleum flow can be obtained without applying some treatment to decrease the viscosity of the petroleum prior to production. Steam flooding has been used successfully in many such reservoirs with varying degrees of success. Injection of steam into a formation raises the temperature of the formation petroleum contacted by the steam, thereby reducing its viscosity and increasing its ability to flow when a sufficient pressure differential exists within the formation to move heated petroleum toward a production well where it can be recovered to the surface of the earth.
Steam injection has been utilized for recovering viscous petroleum from subterranean deposits in a number of different processes. In one type of steam stimulation, steam is injected into a well, the well is shut in for a period of time, and then production is taken from the same well as was used for steam injection. This method is commonly referred to as cyclic steam injection or huff-puff steam stimulation. In another general class of steam-stimulated viscous oil recovery methods, steam is injected into a formation via one or more injection wells to displace petroleum through the formation toward a remotely-located well where it is recovered from the formation and produced to the surface of the earth. This second type of steam stimulation is referred to as steam drive.
Both of the above-described steam stimulation techniques require that the formation's steam injectivity be sufficiently high to permit injection of a minimum quantity of steam into the petroleum formation in order to raise the temperature of the petroleum, thereby reducing its viscosity sufficiently that it will move through the formation under the pressure differential imposed by the steam injection. When steam is injected into a subterranean reservoir containing viscous petroleum, the petroleum viscosity is decreased to a point where it will begin to migrate and thereby form a oil bank in the formation. An oil bank is a zone within the formation having a higher oil saturation than the original oil saturation, moving in the general direction of petroleum production well.
Certain formations have been found in which steam stimulation is not effective because the formation has very low steam receptivity. These Formations are characterized by high oil viscosity, high oil saturation and are usually fully liquid filled. Even if some steam can be injected at first, the oil bank formed begins to cool at its leading edge as it migrates away from the injection well, thereby resulting in the formation of a high viscosity oil bank which becomes immobile within the formation a short distance from the injection well. Once this occurs, further steam injection is not possible because the high oil saturation in the oil bank reduces the permeability of that portion of the formation which greatly reduces steam injectivity. Once the cooled oil bank forms, it becomes impossible to decrease the viscosity of the immobilized viscous oil bank by contact with steam because no more steam can be injected into the formation.
The above described problem has been recognized by persons experienced in oil recovery procedures, and numerous techniques have been described for improving injectivity of steam into formations containing relatively high oil saturations of very high viscosity petroleum. One of the classical methods for increasing the ability of a formation to accept injected fluid is fracturing, but it has been determined that in the formations such as those described above, fracturing of a formation prior to injection of steam is not a satisfactory solution. While steam will move into the formation through the fractures, as it warms the high viscosity petroleum in the portions of the formations adjacent to the openings created by the fracture process, the viscosity of the petroleum is reduced sufficiently to allow the petroleum to flow into the fractures where it is displaced away from the injection well by the injected steam. As the fluid moves ahead of the steam, it cools and again becomes immobile, closing off the fracture flow path (so long as the injection pressure is less than the fracture pressure) thereby resulting in the same problem as was obtained prior to the fracturing of the formation.
It has also been disclosed in certain prior references that injection of a non-condensable fluid into the formation prior to or simultaneously with the steam injection will maintain flow channels open sufficiently to permit continuing injection of the steam into the formation for successful steam drive viscous oil recovery. The non-condensable gas does indeed open up certain flow channels which permits deeper penetration of the steam into the formation initially, but the heated oil moves into these flow channels much as was described above for results obtained when the formation is fractured, and the flow channels are soon plugged with the viscous petroleum.
In view of the above discussion, it can be appreciated that there is still a substantial unfulfilled need for a method for treating a subterranean formation having very low steam injectivity because of high content of very viscous petroleum to permit the successful development of a steam drive oil recovery process within the formation which does not result in the formation of a flow-impeding barrier within the formation as the viscous petroleum cools and becomes immobile.